idr test suite: Take RCU read lock in idr_find_test_1

stable inclusion
from stable-5.10.31
commit 9a7552daa93bf38f6975cb47f6f726d3e2b81a60
bugzilla: 51792

--------------------------------

[ Upstream commit 70358641 ]

When run on a single CPU, this test would frequently access already-freed
memory.  Due to timing, this bug never showed up on multi-CPU tests.

Reported-by: Chris von Recklinghausen <crecklin@redhat.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by:   Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>